Surgical procedures involving fasteners on skeletal structure presently suffer from several common frailties. One is the inability to accurately orient the fastener during insertion to keep the fastener from wandering. Bone interiors have a sponge like (cancellous) texture. Even with a predrilled pilot hole, fasteners still frequently skew off the axis of the pilot hole, making it difficult to register the fastener at its distal end with another instrumentality.
Another frailty is the failure to provide a reliable thread portion that engages the bone of the patient in a manner which retards its working loose. Bone is a remarkable structure which varies both in hardness and elasticity as a function of both age and location. Loads on the fastener must accommodate not only these constraints but also the dynamics of forces generated by the patient in daily activities.
The buttress thread is presently the industry standard. Unfortunately, buttress threads are suitable for use only where load forces on the fastener are applied in one direction. (Bhandari, Design of Machine Elements (2007), page 204). Where the load forces are multidirectional or not unidirectional and axial, failure can occur. One manifestation of buttress thread failure is “toggling” where the fastener works on the bone and enlarges the hole within which the fastener resides.
Yet another frailty related to high insert torque makes it impossible to insert the screw through bone fragments without it grabbing the fragment and causing it to rotate with the screw causing significant trauma to soft tissues and a failed fixation of the fragment.
In an attempt to offset the multiple problems of buttress threads, it is common practice to design the buttress thread profile to increase retention by increasing friction. Increased friction leads to elevated temperatures during insertion, potentially damaging bone tissue. A temperature excursion greater than 116 degrees Fahrenheit (47 degrees Celsius) at the insertion site causes osteonecrosis which cannot be repaired and which the body cannot heal. And, even worse, excess heat compromises the fastener's ability to remain in place since the bone has died at the screw thread interface. As a consequence, some procedures rely on liquid cooling at the site during the drilling/insertion procedure, but, even then, the process generates so much heat that the heat generating tool can be too hot to touch because the liquid cooling is merely topical.
Since the only variables (thread pitch, crest and root diameters) in the buttress thread design are interrelated, improving retention increases friction and insertion torque, resulting in heat generation and impairing the surgeon's ability to feel the insertion torque of the fastener in a meaningful way. This still leaves the fundamental problem of the thread's inability to withstand multidirectional forces unaddressed, while creating several new problems.
The surgeon has no tactile feedback when inserting the fastener. The buttress fastener is harder to start and is prone to stripping, especially when used in conjunction with a plate. The buttress thread induces a radial force, perpendicular to the fastener's long axis, which increases the probability the bone will split or crack, making a procedure much more difficult, if not impossible. The buttress threaded fastener can wander during insertion, making its registry with an instrumentality at the distal end of the fastener a recurring problem. Wandering away from the pilot hole often results in cutting new threads in an unintended location or stripping out the pilot hole which retards healing and actually induces trauma; and, where there is a temperature excursion above 116 degrees Fahrenheit, it can also cause osteonecrosis of the adjacent bone.
The present invention's thread geometry minimizes insertion force. This allows the surgeon tactile feedback and reduces the effort required to deploy the fastener. This, in conjunction with a forward chip feeder, a centering pilot, and improved thread cutting features keep friction low, the fastener aligned and directed from wandering away from the preferred path.
The distal end of many fasteners includes a relief area (flute) designed to help cut through bone, defining a “self-tapping” fastener. Buttress thread fastener's self-tapping features a flute that is straight or at least close to in-line with the axis of the screw. That is, as the fastener is advanced, the cutting edges send the bone chip towards the head of the fastener which is into the path of the helical threads. This bone debris accumulates along the thread teeth and increases insertion torque and friction which therefore generate additional heat. The debris also makes the fastener harder to insert and provides a poor interface with the bone and the fastener.
In the present invention, the cut chips curl away from the cutting edges and are fed forward into the predrilled pilot hole in advance of the fastener because the flute is “left handed” on a right handed threaded screw. That is, as the fastener advances, the flute forces the chips forward into the pilot hole ahead of the screw. This results in exact clearance between the fastener and the portion of the bone being formed as “bone teeth” (that portion which engages the threaded fastener). The interface between the bone and fastener is therefore substantially free of the cuttings and provides healthier bone tissue adjacent the fastener to prevent additional trauma to the bone.
Another problem associated with buttress style threads is that the area between threads of the fastener is the only site of anchoring in the bone, and because of the design constraints associated therewith, this site is difficult to optimize. Stated alternatively, the metal of the fastener is orders of magnitude stronger than the retaining bone so that when failure occurs, it always involves bone trauma.
The present invention maximizes the bone being engaged while minimizing the fastener's thread, something impossible to do with a buttress thread and other common threads and common manufacturing processes. The result is less bone trauma and less bone removed to increase bone strength to retain the fastener better.
This invention discards conventional thinking and manufacturing processes in the pursuit of new and desirable functions that can be achieved from the thread profile.
Heretofore manufacturers have adopted a simple and very fast manufacturing process that produce screws that function no better than common wood screws.